A diode-type electron gun is used to investigate the DC emitting characteristics of a single crystal LaB6 thermionic cathode. The emitting area is a planar 2-mm-diameter, achieving transmitted currents up to 175 mA and the current density 5.57A/cm2. Comparing the measured current with Longo equation, which is accurate and applicable in the regime of practical operation. The circumstance has an effect on the emitting ability of cathode, when the vacuum is more than 7×10-4 Pa, and the current density apparently becomes lower. Aggradation and diffusion of LaB6 to pyrolytic graphite make the resistance of the heater lower. Experimental results show that DC emitting current density is lower than pulsed emitting due to the finite diffusion velocity of atom La and the surface adsorption of remainder-gas.